This invention relates to optical pulse generators, and more particularly to optical pulse generators which generate very short high repetition rate optical pulses for high speed data communications and signal processing applications.
In order to transmit digital signals over optical fibers at the high Gbit/sec speeds which the optical transmission medium is capable of handling or for signal processing at such high speeds, a source of very short, picosecond, high repetition rate optical pulses is required. For example, for optical transmission on an optical fiber, an optical pulse generator is needed to provide an optical pulse stream of very short high repetition rate optical pulses to gating circuitry which gates or blocks each pulse in the stream to the fiber in accordance with the "1" and "0" bits in the data stream to be transmitted. Laser mode-locking is a well known method for generating such very short high repetition rate optical pulses. A brief discussion of laser technology and mode-locking is presented for better understanding of the present invention.
A typical laser consists of two essential elements: gain and feedback. A beam of light passing through the gain, or amplifying, medium stimulates it to release its stored energy in the form of additional light that adds to, or amplifies, the beam. Feedback is achieved by placing the gain medium within a resonator (a set of mirrors that reflects the beam back and forth through the gain medium). As a results of this cumulative process, an intense coherent beam of light is produced. The light from such a laser is composed of a number of discrete wavelengths corresponding to different resonant frequencies, or modes, of the resonator.
The total output of such a laser as a function of time will depend on the amplitude, frequencies, and relative phases of all of these oscillating modes. If there is nothing which fixes these parameters, random fluctuations and nonlinear effects in the laser medium will cause them to change with time, and the output will vary in an uncontrolled way. If the oscillating modes are forced to maintain equal frequency spacings with a fixed phase relationship to each other, however, the output as a function of time will vary in a well-defined manner. The laser is then said to be mode-locked. The form of this output will depend on which laser modes are oscillating and what phase relationship is maintained.
More precisely, mode-locking is achieved when all of the laser's resonant longitudinal modes are generated in phase to produce a Fourier sum. This yields a series of pulses having a repetition rate corresponding to mc/2L where c is the speed of light in a vacuum, L is the cavity length and m is an integer. Such mode-locking is achieved by supplementing the internal laser cavity with an external mirror cavity. A mirror is disposed a particular distance from the crystal so that the combined cavity length is L. The external mirror provides a reflective surface for photon resonance.
It has been demonstrated that very short optical pulses can be generated via mode-locking using a fiber as the coupled external cavity. In particular, G. Eisenstein, R. S. Tucker, S. K. Korotky, U. Koren, J. J. Veselka, L. W. Stulz, R. M. Jopson, and K. L. Hall, in "Active modelocking of an InGaAsP 1.55 .mu.m laser in a fiber resonator with integrated single mode output port," Electrn. Lett. 21 pp. 173-175, 1985, and the present inventor in "Generation of self-sustained optical pulses from transients in external cavity semiconductor laser", in Electrn. Lett. 24 pp. 137-139, 1988, have shown that lensed, single-mode, fiber cavities with a partially reflecting end can be employed successfully for mode-locking and pulse generation. In these arrangements, the end reflectively is usually provided by metalization or by a multilayer dielectric coating. Disadvantageously, in any of these arrangements, once the cavity is fabricated and coupled to the laser, the cavity length and its end reflectivity cannot be adjusted and none of the parameters of the pulse stream, such as pulse width, pulse amplitude and pulse repetition rate, which are functionally related to the length and the reflectivity of the external cavity, can be varied.
An object of the present invention is to provide an external cavity to the laser in which the parameters of the pulse stream can be varied by either mechanical or electrooptic adjustments of the external cavity parameters.